added variable to set project path in .env file

.gitignore

@@ -1,2 +1,3 @@
 *.swp
 *~
+.env

README.md

@@ -1,3 +1,12 @@
 # jekyll-dev
 
-Simple container to help develop Jekyll sites.
+Simple container to help develop [Jekyll](https://jekyllrb.com/) sites.
+
+## Usage
+
+You need a `.env` file which sets the project path:
+```sh
+cd /path/to/jekyll/project
+echo "PROJECT_PATH=/path/to/jekyll/project" > .env
+docker-compose up -d
+```

docker-compose.yml

@@ -6,7 +6,7 @@ services:
     build: .
     container_name: jekyll-dev
     volumes:
-      - /home/david/Projects/just-the-docs/docs:/mnt
+      - ${PROJECT_PATH}:/mnt
     ports: 
       - 127.0.0.1:4000:4000
     restart: unless-stopped